The invention relates to compressible fluid type spring devices for vehicle suspensions. More specifically, the invention relates to airsprings of the type with a flexible type rubber sleeve having rolling lobes.
Airsprings having a sleeve for forming a closed chamber to receive a pressurized fluid are well known. Usually, the airspring sleeve includes a corded fabric or a nylon cord to strengthen the sleeve and retain the sleeve shape.
Rolling lobe type airsprings are well known in the art and are made with a sleeve having a chamber portion connected to a closure memeber and an inverted rolling lobe portion connection to a piston that partially reciprocates in the chamber portion of the sleeve. The general formula for calculating a spring rate of such an airspring is well known and documented such as in U.S. Pat. No. 4,629,170.
An airspring is a load support member that utilizes the compressible characteristics of air for a spring effect. One method of changing the spring rate of an airspring is to change the effective area that is acted on by the internal pressure of the spring. This is done by altering the external shape of the piston which laterally supports part of the rolling lobe portion of the sleeve. Theoretically, there is no change in effective area or spring rate if the piston is straight sided or cylindrical. However, a reduced effective area is achieved by a frustoconical shaped piston that reduces size as it enters the chamber portion; and an increase in spring rate is achieved by a frustoconically shaped piston whose size increases as it enters the chamber portion.
In use, rolling lobe airsprings typically encounter angular displacements or torque which result in ride harshness. The ride harshness is pronounced at low amplitude undulation of the vehicle. Rolling lobe airsprings having one piston and one rolling lobe can only compensate for very small angular displacements. Also, rolling lobe airsprings having two substantially cylindrical pistons and two rolling lobes, due to the increased amount of piston travel, can only compensate for very small angular displacements.
In accordance with the invention, an airspring and method are provided. The airspring has a construction that can accommodate large angular displacement of the vehicle suspension. The airspring construction can also be used to vary the spring rate associated with a piston entering and exiting an air chamber. An airspring is provided which has a sleeve having two rolling lobes and two pistons.
The first rolling lobe is connected at an end of the first rolling lobe to a first piston. The second rolling lobe is connected at an end of the second rolling lobe to a second piston. Each of the first and second pistons includes a rolling surface that in conjunction with the first and second rolling lobes defines an effective area for each of the first and second pistons. The minimum effective area of the second piston is less than the minimum effective area of the first piston and the maximum effective area of the second piston is greater than the maximum effective area of the first piston.
In a preferred embodiment of the invention, the first piston is substantially frustoconical and the second piston is substantially frustoconcical. In an alternate embodiment, the geometric shape that determines the effective area need not be symmetrical about any axis and may describe any contour as required by a user.
An object of the invention is to provide an airspring design that includes two pistons and two rolling lobes with one of the pistons performing a majority of the longitudinal displacement.
An advantage of the invention is that large angular displacements of the vehicle suspension can be accommodated.